Carboxylic acid anhydrides, Lewis Acids, and boron trifluoride:amine complexes are curing agents that have been found to be useful with epoxy resins for insulating applications, as described by Lee and Neville in the Handbook of Epoxy Resins, McGraw Hill, 1967, pages 11-1 to 11-8 and 12-1 to 12-27. Usually, the addition of an accelerator is required to give reasonable gel times at elevated temperatures, but at room temperature, even with high concentrations of accelerators, very slow gel times are experienced. Considerable effort has been devoted in recent years to developing improved room temperature curing agents for epoxy-anhydride resins.
Ecke et al., in U.S. Pat. No. 3,114,752, taught the reaction of tetrahydrofuran with maleic acid in the presence of a free radical initiator to produce monomeric 1:1 adducts. Free radical initiators were taught to include ultraviolet light and various persulfates, peroxides and nitrides. The compounds formed were bonded adducts rather than disassociated species such as free radicals, and were taught as useful plasticizers and curing agents for epoxy resins. Smith et al., in U.S. Pat. No. 4,273,914, discovered a low temperature, fast curing epoxy insulating composition, which consisted of an epoxy resin and a carboxylic acid anhydride complex. The anhydride complex was made by the low temperature reaction of a selected Lewis Acid catalyst, such as antimony pentrachloride, titanium tetrachloride, boron trifluoride, tin tetrachloride, or triphenyl tin chloride, with a carboxylic acid anhydride. There, the catalyst and anhydride were simply pre-reacted at a reacting mass temperature of from 10.degree. C. to about 45.degree. C. The complex allowed substantially complete cure of the epoxy resin at 25.degree. C. in about 48 hours.
Von Brachel et al., in U.S. Pat. No. 3,499,007, utilized a peroxide initiated, non-irradiated, free-radical chain reaction of maleic anhydride and straight chain polyalkylene ethers, at from about 80.degree. C. to 160.degree. C., to provide addition products, noting that the literature showed successful reaction of maleic anhydride with tetrahydrofuran, but not dioxane, in the presence of radical initiators. These addition products were found useful as raw materials for lacquers, and as surface active anhydride components in the production of polyesters. These addition products were usually reacted at from 100.degree. C. to about 130.degree. C. with epoxies and the like.
Charge-transfer systems have recently been shown capable of polymerizing monomer and epoxy resins. Williamson et al., J. Polm. Sci., Polm. Chem. Ed., Vol. 20, pp. 1875-1884, 1982, "Laser-Initiated Polymerization of Charge-Transfer Monomer Systems" describe polymer formation after laser exposure in three successful systems: 9-vinylanthracene/diethylfumarate; 2-vinylnaphthalene/fumaronitrile, in methylene chloride solvent; and 2-vinylnaphthalene/fumaronitrile, in sulfolane solvent. Another article, "Laser Initiated Polymerization of Charge Transfer Monomer Systems: Copolymerization of Maleic Anhydride with Styrene, Vinyltoluene and t-Butylstyrene", by R. K. Sadhir et al., Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem., Vol. 23 No. 1, pp. 291-292, March 1982, describes vinyl-maleic anhydride systems and a theoretical discussion of 3,600 Angstrom Unit laser irradiation of such systems to form charge transfer systems.
Later articles, "Laser-initiated Copolymerization of Maleic Anhydride with Styrene, Vinyltoluene, and t-Butylstyrene", by R. K. Sadhir et al., J. Polym. Sci. Polym. Chem. Ed., Vol. 21, No. 5, pp. 1315-1329, May 1983, and "Laser-Initiated Polymerization of Epoxies in the Presence of Maleic Anhydride", by R. K. Sadhir et al., J. Polym. Sci. Polym. Chem. Ed., Vol. 23, pp. 411-427, 1985, give a more detailed description of laser-initiated polymerization of styrene, vinyltoluene and t-butystyrene in the presence of maleic anhydride, and laser-initiated polymerization of cyclohexene oxide in the presence of maleic anhydride, respectively.
Sadhir et al., in U.S. Patent Application Ser. No. 731,745, filed on May 7, 1985, utilized a reactive, irradiated catalytic complex as a low temperature curing agent for organic resins. The complex was produced by U.V. or laser irradiating a mixture of carboxylic acid anhydride and at least one of a cyclic compound selected from tetrahydrofuran, dioxane, trioxane and sulfolane, with no use of catalysts or initiators. Another application in the area is Sadhir et al., U.S. Patent Application Ser. No. 703,165, filed on Feb. 19, 1985, which used additional catalysts.
Sadhir et al., in U.S. Patent Application Ser. No. 739,242 filed on May 30, 1985, cold concentrated these irradiated catalytic complexes to improve reactivity. These concentrated catalytic complexes were described as sole room temperature catalysts with epoxy resins and vinyl monomers, to provide impregnating, potting, or protective encapsulating resins for motor coils, or coil connection insulators for high voltage rotating apparatus. Examples showed a quick room temperature cure with cycloaliphatic epoxy resins. It had been found, however, that these complexes provided a slower room temperature cure with bisphenol A epoxy resins than with cycloaliphatic epoxy resins.
Since the bisphenol A epoxy is the most commonly used and inexpensive type of epoxy resin, it is highly desirable to find a fast acting catalyst for them which is useful at room temperature, and to provide fast, room temperature curable bisphenol A epoxy coating compositions. It would also be highly desirable to be able to fast cure cycloaliphatic epoxy resins at times below 3 minutes at room temperature, for fast production line, thin coating of a variety of articles.
As a further improvement, Saunders et al., in U.S. Patent Application Ser. No. 926,304, filed on Nov. 3, 1986, utilized a boron trihalide complex in the concentrated catalytic complexes of Sadhir et al., to lower room temperature cure time. Inclusion of such complexes tended however to lower electrical properties of the cured composition somewhat. Additionally, in the cases involving concentrated catalytic complexes, crystallization in the solution can occur in the range of about 10% to 15% concentration after 6 hours to 10 hours, limiting storage life and mixing ability. What is needed is a means to concentrate the catalytic complexes without crystallization and to modify their structure to provide an extremely reactive curing agent for resin systems.